The present invention relates to a process for the epoxidation of olefins, in particular to the working up of the product stream from the epoxidation reaction.
From EP-A 100 118 it is known that propene can be converted with hydrogen peroxide into propene oxide if titanium silicalite is used as catalyst. The reaction is preferably carried out in the presence of a water-miscible solvent in order to improve the solubility of propene in the reaction mixture. Preferably solvents are used that have a boiling point between the boiling points of propene oxide and water in order to be able to separate the solvent from the reaction mixture by a distillation stage and recycle it to the reaction. Methanol is preferably used as solvent.
From U.S. Pat. No. 5,599,955 it is known that the reaction mixture which is obtained in the epoxidation reaction and which consists of propene oxide, propene, possibly propane, solvent and water can be separated by a succession of distillation steps, in which the mixture is separated in a first distillation step into an overhead product containing propene oxide, propene, and possibly propane, and into a bottom product containing the solvent and water. The efficient separation of propene oxide and solvent in a distillation step requires a large number of separation stages and a high reflux ratio in the distillation column.
WO-A 99/07690 describes a process for the purification of a methanol-containing product stream from the epoxidation of propene that also contains acetaldehyde as an impurity. In this case the crude product stream from the epoxidation is subjected to a fractional distillation, in which connection it is particularly important that methanol is present in sufficient amount in the overhead product in order to achieve a substantially complete transfer of acetaldehyde to the bottom product. To this end the concentration of methanol in the overhead product is 2-6 wt. %. A distillation column with 20-60 separation stages and a reflux ratio of between 10:1 and 30:1 is furthermore necessary in order to achieve the best possible quantitative separation of the acetaldehyde. This arrangement accordingly involves high investment and operating costs for the distillation column.
From U.S. Pat. No. 5,849,938 it is known that in the distillative working up of the methanol-containing reaction mixture from the propene epoxidation, the difference in volatilities of propene oxide and methanol can be increased by carrying out the distillation as an extractive distillation using water or propylene glycol as extraction agent. The purpose of this extractive distillation is to separate methanol as well as further high boiling point impurities as quantitatively as possible from the desired product, namely propene oxide, in one distillation step. This requires a large number of separation stages and a high reflux ratio in the distillation column. In order to achieve the desired separation result, at least 10 theoretical trays, preferably 20-60 theoretical trays are required with a reflux ratio in the range from 5:1 to 15:1. The working examples disclose 25 or 50 theoretical trays and a reflux ratio of 9:1 for the extraction distillation column.
WO 00/07965 discloses a process for reacting an organic compound with a hydroperoxide especially for reacting propene with aqueous hydrogen peroxide in methanol as solvent, wherein the reaction product is subjected to a distillation step to separate a head product comprising propene, propene oxide and methanol from a bottom product comprising unreacted hydrogen peroxide. The distillation column having 15 theoretical trays is run at ambient pressure. The bottom stream is used as feed stream for a second reaction step.
In the known processes for the epoxidation of propene with H2O2 and titanium silicalite followed by distillative working up of the reaction mixture, due to the large number of separation steps and the high reflux ratio the residence time of propene oxide in the sections of the distillation column is long and there are also high concentrations of water and higher boiling point byproducts in the said sections, and accordingly the temperature is considerably higher than the boiling point of propene oxide under the distillation conditions. It has now been found that, as a result, there is an increased level of secondary reactions of propene oxide with water and other substances containing hydroxyl groups in the reaction mixture, which leads to undesirable losses of propene oxide. This disadvantage is particularly serious if the distillation is carried out under elevated pressure and thus at elevated temperature, which is advantageous for industrial exploitation since propene oxide can then be condensed with cooling water at the head of the distillation column and no expensive and energy-intensive cooling units have to be used.
This disadvantage of the known processes is exacerbated still further if the titanium silicalite catalyst used for the epoxidation reaction passes together with the reaction mixture into the separation column, since the catalyst also accelerates the undesirable secondary reactions of propene oxide with water and/or with other substances containing hydroxyl groups. If the epoxidation reaction is carried out with a suspended titanium silicalite catalyst, then in the known processes this catalyst must therefore be removed completely from the reaction mixture before the distillative separation of propene oxide and solvent. The separation of the catalyst at this point is particularly complicated since the separation is carried out in the presence of the highly volatile and carcinogenic substance propene oxide, and expensive and complicated industrial safety measures are therefore necessary. Also, precautions have to adopted in the known processes when using a fixed bed catalyst, for example by employing filtration, in order to prevent catalyst abrasion products settling in the distillation column and thereby causing product losses on account of the catalysis of the secondary reactions of propene oxide with water and/or other substances containing hydroxyl groups.
EP-A 1122248 discloses a process for the working up of a product stream from the epoxidation of olefins that contains olefin, olefin oxide, water-miscible organic solvent, hydrogen peroxide and water, by separating this product stream into an overhead product containing olefin, olefin oxide and organic solvent, and into a bottom product containing organic solvent, hydrogen peroxide and water, whereby the separation is carried out in a pre-evaporator with at most 5 theoretical separation stages at a pressure of 3 to 8 bar and 20 to 60% of the total amount of organic solvent introduced with the product stream is removed with the overhead product and the residue remains in the bottom product.
Although the process described in EP-A 1122248 results in an considerable reduction of losses of epoxidation product compared to the hitherto known processes there is still a desire in industry, and accordingly an object of this invention, to further improve the efficiency of the working up of the product stream from the epoxidation of olefins.
The above and other objects of the invention can be attained by a process for the working up of a product stream from the epoxidation of olefins that contains olefin, olefin oxide, water-miscible organic solvent, hydrogen peroxide and water by separating this product stream into an overhead product containing olefin, olefin oxide, and organic solvent, and into a bottom product containing organic solvent, hydrogen peroxide and water, wherein the separation takes place in a pre-evaporator with less than 10 theoretical separation stages and 20 to 60% of the total amount of organic solvent entrained in the product stream is removed with the overhead product, the balance remaining in the bottom product, whereby the separation in the pre-evaporator is conducted at a pressure from 1.5 to less than 3 bar, preferably 2 to 2.9 bar.
This object is furthermore achieved by a process for the catalytic epoxidation of olefins in which the olefin is reacted in a reaction step with aqueous hydrogen peroxide in a water-miscible organic solvent in the presence of a titanium silicalite catalyst, the product stream from the reaction step being optionally added to a pressure release step and then worked up according to the aforedescribed process without prior distillative separation.
It has now been found that in the epoxidation of olefins with hydrogen peroxide and a titanium silicalite catalyst using an organic water-miscible solvent, the losses of olefin oxide in the distillative working up of the reaction mixture can be further reduced compared to the process disclosed in EP-A 1122248 if the processes according to the invention are employed. Furthermore it has been surprisingly discovered that by conducting the pre-evaporation within the pressure range of the process of the present invention decomposition of unreacted hydrogen peroxide is considerably reduced. Consequently the generation of molecular oxygen by peroxide decomposition is reduced. Therefore the likelihood of generating explosive compositions in any of the subsequent work-up stages is very low so that no extra precautions have to be taken to ensure safety of the process. Thus this measure improves the overall economics of the process.
A further advantage of the present process is, that the bottom stream comprising unreacted hydrogen peroxide from the pre-evaporation can be partially recycled to the reaction step.
Still a further advantage of the present invention compared to the teaching of EP-A 1122248 is the improved energy efficiency as will be discussed in some detail below.